For the past 17 million years and perhaps longer, motion of Earth's tectonic plates has moved California west, away from Utah and Arizona.
As the area in between grew wider, Earth's crust cracked along major faults to form the mountains and valleys of the Basin and Range Province that today cover Nevada and western Utah. In the early stages of this process, large amounts of molten basalt intruded Earth's crust in northern Nevada, forming a linear, near-vertical dike swarm over 500 km long.
This structure -- named the "northern Nevada rift" -- was interpreted to record roughly southwest-oriented stretching during early growth of the Basin and Range, which changed to a more northwesterly direction around 10 million years ago. This paper by Joseph P. Colgan and colleagues presents new and existing data that instead suggest that Basin and Range stretching has always been oriented west or slightly northwest.
This direction is consistent with formation of the Basin and Range by "collapse" of a formerly high plateau -- somewhat like northern Chile today -- that once occupied what is now the state of Nevada, while the orientation of the northern Nevada rift may have been controlled by unusual, short-lived conditions associated with the beginning of the Yellowstone hotspot.
Paper: Joseph P. Colgan et al., U.S. Geological Survey doi: 10.1130/G33512.1